Electric motors generally include a stator and rotor mounted for rotation relative to the stator. An electromagnetic drive system including a plurality of magnets and/or electromagnets on the rotor and stator is used to drive rotation of the rotor relative to the stator. The rotor is connected to an output shaft so that as the drive system rotates the rotor the output shaft rotates. Operation of the motor, particularly under a load, generates heat. In some cases, one end of the motor (e.g., the driving end) reaches a higher temperature than the other due to this heat generation. The electromagnetic drive system also generates heat. Heat associated with operation of the motor can promote premature breakdown of lubricants (e.g., in the bearings), damage the electromagnetic drive system, and otherwise interfere with desired operation of the motor.
Some electric motors include passive cooling features, such as cooling fins and the like, to facilitate heat transfer out of the motor. Some electric motors include active cooling systems, such as a forced air ventilation systems. For example, a fan can be attached to the output shaft so rotation of the output shaft rotates the fan to generate air flow to cool the motor. Active cooling systems and passive cooling features such as cooling fins can be used in combination.
A motor's stator and rotor are commonly mounted in a housing. The housing provides a frame for anchoring the mounted rotor and stator and holding the stator fixed relative to the housing. The housing can also be a barrier preventing people (or other objects) from contacting parts of the motor inside the housing. In some cases the stator and rotor are totally enclosed by and sealed within the housing in order to limit the potential for dust and other debris to interact with the rotor or stator and thereby interfere with operation of the motor. A fan can be used to cool a totally enclosed motor (e.g., by directing air over the housing), in which case the motor may be referred to as Totally Enclosed Fan-Cooled (TEFC). The drive end of a TEFC motor is typically hotter than the opposite end because the fan is installed opposite the drive end. Sometimes, an internal air circuit is used to improve heat distribution in the motor by interchanging air from one end of the motor to the other. For example, in one conventional TEFC motor, a fan pumps air from one end of the housing to the other through passages in the rotor. Air is returned to the first end of the housing through passages in the stator and/or housing.